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<h4>Known Problems of the C/C++ Preprocessor</h4>
<div>
	Preprocessor metaprogramming is subject to heated discussions.&nbsp;
	Part of this is caused by bad experiences with dangerous techniques,
	such as defining inline functions using macros.&nbsp;
	As a rule of thumb, if you can find a clean and manageable way to do something
	without the preprocessor, then you should do it that way.
</div>
<div>
	Let's survey some of the widely known problems of the preprocessor in a problem/solution format.
</div>
<h4>Problem #1</h4>
<div>
	The preprocessor does not respect scope, therefore macros can accidentally and sometimes silently replace code.
</div>
<div>
	<b>Solution A</b>
	<div>
		Use all caps identifiers for macros and only macros.&nbsp;
		This practically eliminates the possibility that a macro might replace other kinds of code accidentally.
	</div>
</div>
<div>
	<b>Solution B</b>
	<div>
		Use the local macro idiom:
	</div>
<div class="code"><pre>
#define MACRO ...
// use MACRO
#undef MACRO
</pre></div>
	<div>
		This makes sure that a macro cannot accidentally replace code outside of the scope of the local macro.
	</div>
	<div>
		A problem with this solution is that the #undef cannot be automated and may be forgotten.&nbsp;
		Experienced programmers generally write the #undef either immediately before (in time)
		or immediately after writing the macro definition.
	</div>
</div>
<div>
	<b>Solution C</b>
	<div>
		Use the unique macro prefix idiom.
	</div>
<div class="code"><pre>
#define UMP_MACRO
// use UMP_MACRO
</pre></div>
	<div>
		This makes accidental substitution and collisions highly unlikely.&nbsp;
		Problems with this solution include:
	</div>
	<ul>
		<li>There can still be naming collisions inside a large project.</li>
		<li>Macros still pollute the global namespace.</li>
	</ul>
	<i>By combining all solutions, whenever possible, the scope problem can be largely avoided.</i>
</div>
<h4>Problem #2</h4>
<div>
	Preprocessor code is difficult to read.&nbsp;
	It requires an understanding of the basic process of how the preprocessor recursively expands macros,
	finding macro definitions, and mentally substituting the parameters of the macro.
</div>
<div>
	<b>Solution</b>
	<div>
		Any kind of programming requires a basic understanding of how the code is executed.&nbsp;
		Any parameterization technique, including simple functions and templates requires finding
		the definition and mentally substituting parameters.
	</div>
	<div>
		However, it is good to know a few techniques:
	</div>
	<ul>
		<li>By using as many local macros as reasonable, the bulk of the searching process can be eliminated.</li>
		<li>Code browsers and text search tools make it easier to find the definitions.</li>
		<li>The compiler can be used for generating the preprocessed source code in order to look for bugs.</li>
		<li>
			Before turning something into a preprocessor metaprogram, first implement a small scale version
			of it without the preprocessor.&nbsp;
			The work bottom-up, replacing hand-written constructs by using the preprocessor.&nbsp;
			This way you can test the code incrementally.&nbsp;
			Experienced programmers often skip many stages, but if something proves too complex to write
			directly, it is always possible to fall back to incremental methods.
		</li>
		<li>
			If you insert a special symbol into the preprocessor code in places where there should be a line break,
			you can make code readable after preprocessing simply by using a search and replace tool.
		</li>
	</ul>
	<i>An especially important thing to remember is to limit the use of the preprocessor to
	structured, well-understood, and safe methods.&nbsp;
	Structure helps to understand complex systems <a href="../bibliography.html#mcconnell">[McConnell]</a>.</i>
</div>
<h4>Problem #3</h4>
<div>
	"I'd like to see Cpp abolished." - <i>Bjarne Stroustrup</i> in <a href="../bibliography.html#stroustrup">[Stroustrup]</a>.
</div>
<div>
	<b>Solution</b>
	<div>
		The C/C++ preprocessor will be here for a long time.
	</div>
	<i>In practice, preprocessor metaprogramming is far simpler and more portable than template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a>.</i>
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